Radiation was delivered to tumors as a single dose of 20 Gy or fractionated 10 Gy doses delivered at time 0 and 24 hrs later (minimum pO2) or 72 hours later (maximum pO2). Tumor regrowth delay assays were used to compare the effectiveness of fractionated radiation given under "hypoxic" or "oxic" conditions. Significant differences in the time to reach 4X the treatment volume were observed. Radiation was more effective when the second dose was timed to correspond with maximum re-oxygenation after the initial dose.